Circuit arrangement for use with widely separated frequency bands



y 5, 1949. J. D. HOLLAND 2,474,978

CIRCUIT ARRANGEMENT FOR USE WITH WIDELY-SEPARATED FREQUENCY BANDS FiledAug. 18, 1945 3 Sheets-Sheet l Inventor Town bcuolms HaLLfi D Filed Aug.18, 1945 i I I I July 5, 1949. J. D. HOLLAND 2,474,978

CIRCUIT ARRANGEMENT FOR USE WITH I WIDELY-SEPARATED FREQUENCY BANDS 3Sheets-Sheet 2 I nventor \Temn Dauemecu-n9 1 July 5, 1949. J. D. HOLLAND2,474,978

CIRCUIT ARRANGEMENT FOR USE WITH WIDELY-SEPARATED FREQUENCY BANDS FiledAug. 18, 1945 3 Sheets-Sheet 3 Inventor Inm mOuGLfiS New;

I Attorne Patented July 5, 1949 CIRCUIT ARRANGEMENTS FOR USE WITH WIDELYSEPARATED FREQUENCY BANDS John Douglas' l lolland, London, England;assignor, byniesne assigiiments, to International StandayardElectric'Corporation, New York, N. Y., a

corporation xofDelaWar-e A ses es Au ust 18, 1945, Serial'No. 611,378

"" In' Great Britain September 1, 1944 The present invention relates tothermionic valve circuits and particularly to circuits for translatingor generating signals occupying two difi'erentbands of frequencies. Y

"=Itis of-course well-to amplify all the channels of a multiechannelsystem simultaneously' in the-same amplifiers and toseparate thechannels afterwards Joy-means of filters. In such a case Waves of-al'lf-requencies are-simultaneously applied to the inputelectrodes-of anyvalve in the amplifier, and the-amplifiedwaves areall' simultaneouslyderived from't'he output electrodes thereof -I '1 I ln -thecase of thepresent-invention; the amplifier or other translating device is providedwith a multi-electrode'valve haying-at least three grid electrodesbetween the" cathode and 1 the anode which is treated as 'two -tr'iodeshaving a common cathode-and sharing the-same electron stream; Oneofthese-component triodes is used to translate. or generate waves"occupying one band of frequencies; andthe other to translate orgenerate those occupying another band.

In this way=the two channelsare kept-separate, though they are dealt:with by thesame-valve.

It is also well-known to employ a- -pentagrid= valve. as ra -frequencychanger =for a single band of. frequencies"'byvusingnthecathode and thefirst two grids as a triode oscillator andthe-third' grid and:anodewitmthe cathodesas a modulator. Thus Whereas; in: thiszarrangementthe valve is employed only "tov "dear with oneband. of signalfrequencies; according to the'rpresent invention the'two parts-oflthevalve are each employed for separate-bands-offrequencies. w The several:features-.- ofathe: invention are set out asuclaimsl-,.2rand3.respective1y, of the statem nt ofclaim.;-':r1121, a: 'Ihea'inventionwill be described with reference to the-accompanying drawings: in whichFigS.lIfTa1Ild1i2 show simplified schematic circuit diagrams ofarrangementsaccording. to. the

invention; m Y w Fig. 3 shows aschematici-circnit diagram of atWc-pathradio receiver according to the invention. "Fig. 4 shows amodification:of; partofFig. 3..

, .Fig. 5 shows a radio receiver according to the invention providingtwosepara-te signal channels;and'-" w Fig. 6 shows a. double-oscillatoraccording to the invention.

The type of, multii-gridvvalve shownvin the above circuits for thepurpose of illustrating the inventionviscne sometimesicalledtaipentagrid,

having five grids. interposed between the cathode and the anode. Thethird and fifth grids are. connected together to form a single screenelectrode for th-eiourth grid. For thepurposeof the present inventionthe.-.screen. electrode.is not, essential. andv can be omitted, theminimum re.- quirement being that there. should be at least three gridsbetween. the cathode and the anode. There may, of course-,bany number ofgrids in: excess of three the extra. grids-being suitably polarized.v

Fig. 1 shows diagrammatically. an arrange:- ment according to theinvention. A pentagrid valve l= comprises: ascathode :2, three grids 4and 5,.a screen grid electrode ,6 and an anodel. The electrodesl, a andi /constitute one triode; portion, of whichelectrodex i serves as theanode, and electrodes 2,15 andl: constitute another trigode portion.The-cathode 2 is connected to the grounded negative terminal .8 of thehigh tension supply, the positiveterminal 9 of which is con-- nected:directly to the screen grid B and to elQQ- trodes 4- and'lathroughimpedance elements Hiand ill-respectively. The control grids ,3 and :5are connected to inputterminals Hand 13 re:- spectively,.and the anodes4- and Tare connectedto output terminals l4: and: 15 through blocking,condensers l6 and ,lil', respectively. A suitable bias network,zpflqm-ayrbeainserted in series with the cathodeas shown. A condenser 2I serves. as a lay-pass condenser shunting .the high tensionsource;..-.. H

The impedance .elements 18 and I}! represent input circuits reachcarrying a separate channel ofi-signals. I Thesignalinput voltages arerepresented bythe symbolsei and as in series with the elements-I8'and.l;9. .The amplified or otherwise translated signals are obtainedat the output terminals M andzla5 thecorresponding output voltagesbein-glEi and E2. as indicated.

Let f1 and f2 be=the signal frequencies corresponding tolthe.inputpvoltages e1 and 62, and suppose that the valve 1 i is employed'asan amplifier. .Then when both. setsof signals are applied to therespective input grids,..the electron stream proceeding from thecathodewill contain variations at both frequencies, so that. amplified. signalsat bothfrequencies will. appear at bothfof the anodes 4. and .L. Theimpedance elements Hil and II accordingly represent any suitable tunedicirc-uitszor filters, or. the like, adapted. to suppress one of-theirequencies andto allow the other to pass. In order that this. maybe achieved without undulyzcomplicating the impedance, elementa-the;-ztwov frequencies f1. and f2 should preferably be widelyseparated; for example, one should be not less than about double theother, and preferably also one frequency should not be a harmonic of theother. Where the signals comprise two bands of frequencies, then thelowest frequency of the upper band should preferably be at least doublethe highest frequency of the lower band.

Furthermore, the two triode portions of the valve I will usually havedifferent mutual conductances (the upper portion having generally thhigher value) and in order to minimise interference the associatedcircuits should preferably be arranged so that the gains of the twoamplifiers do not differ by more than about 10 decibels. As the outputcircuit which operates at the lower frequency will usually have a higherdynamic impedance than that operating at the lower frequency, it isdesirable to operate the triode portion 2, 3, 3, at the lower frequencyso that by associating with it the high impedance output circuit thegains of the two amplifiers will tend to be equalized.

If the two triode portions of the valve 1 are not operatedsimultaneously, then it is not important that the two frequencies f1 andf2 should be very widely separated, or that they should be harmonicallyunrelated, and a larger difference of gain for the two amplifiers can beallowed, since interference does not arise. In any case however, it isdesirable that the valve should be so biassed that neither of theamplifiers are overloaded.

It will be evident that the signals applied at terminals l2 and I3 areindependently amplified and are kept to separate circuits throughout.The two corresponding channels may be used simultaneously or separately.The arrangement is equivalent to two separate amplifiers each with itsown valve, but the two valves are part of the same pentagrid valve. Thevalve I may also be used as a double frequency changer. In this case thestrap between the terminals 22 and 23 shown in the grid connection isremoved and these terminals are connected to a suitable local carrierfrequency oscillator (not shown). The bias circuit 20 is chosen so thatboth sections of the valve I operate as modulators. Then if f is thelocal carrier frequency, side bands fo+f1 can be obtained from the anode4 and sidebands fo-l-fz from the anode I. The circuit elements It! and Hcould then comprise band filters each of which is adapted to select acorresponding side band and to supply it to terminal I 4 or [5. So longas the frequencies f1 and f2 are well separated, the band filtersrepresented by the elements I {I and H need not be very selective.

The two channels may be provided with different local carrierfrequencies by means of a simple switching arrangement, such as thatshown in Fig. 2, in which the elements which are similar tocorresponding elements of Fig. 1 are designated by the same numerals.

In Fig. 2, 24 and 25 represent the sources of the voltages er and 62 atfrequencies f1 and f2. A switch 26 connects the source 24 to the inputterminal l2 in the position shown, and in the other positions itdisconnects the source 24 and instead connects the source 25 to theinput terminal 13. A carrier frequency source 2! of voltage es andfrequency is is connected by the switch 28 when in the position shown tothe terminals 22 and 23 in series with the cathode 2. This will producea side band f3+f1 at terminal 14 of voltage E3. When the switch 28 isoperated to the other position, it substitutes for the source 21 asource 29 of voltage at at frequency ii. If the two switches 26 and 28are mechanically connected so as to be operated together, then a. sideband f4+f2 at voltage E4 will be obtained at terminal I5. Thus by meansof the two switches 26 and 28 it is possible to select either one of thesources 24 and 25 and to provide at the same time a correspondingcarrier frequency. Moreover, the corresponding side bands are obtainedin separate channels.

It will be evident that the arrangement of Fig. 2 can be either amodulator or a demodulator for either channel; and it could be amodulator for one channel and a demodulator for the other. In theremaining figures of the accompanying drawings three different types ofcircuit according to the invention are given as illustrations of thegeneral principles described with reference to Figs. 1 and 2. Thesetypes are as follows:

(1) A circuit in which the two channels which pass through themulti-grid valve or valves are not intended for use simultaneously, butone channel is used for signals covering one part of a frequency bandand the other for those covering another part of the band.

(2) A circuit in which the two channels are completely independentthroughout and may be used simultaneously or separately.

(3) A circuit in which the two triode portions of a multi-grid valve areused as oscillators for generating two different frequencies.

Fig. 3 shows a circuit according to the invention suitable for a radioreceiver adapted to cover a very wide range of frequencies, for example15 to 26,000 kilocycles per second (kc. s.). Such a large range cannotbe satisfactorily covered by the use of a single intermediate frequencyfor reasons of selectivity, image suppression, and gain. It is desirabletherefore to divide the range into two parts, for example 15 to 500 kc.s. using an intermediate frequency of 40 kc. s. and 500 to 26,000 kc. s.with an intermediate frequency of 560 kc. s. According to the presentinvention two channels are provided in the receiver, in whichintermediate frequencies of 40 and 560 kc. s. are respectively derivedand amplified in the two corresponding portions of the valves.

In the radio receiver shown in Fig. 3 the waves are received on anantenna 30 and are passed to the high frequency amplifiers and selectivecircuits 3| which may be arranged in any convenient way. A switch 32enables the output of 3| to be applied to the upper or lower input gridof a pentagrid valve 33 arranged similarly to the valve I of Fig. 2. Acarrier frequency oscillator 34 of adjustable frequency corresponds tothe oscillator 21 of Fig. 1 and is connected to the cathode circuit ofthe valve 33 through a transformer 35. The usual bias network for thiscathode is shown at 36.

Two tuned output transformers 3'! and 38 couple the two triode portionsof the valve 33 to the corresponding triode portions of an amplifyingpentagrid valve 39. This amplifying valve has the usual cathode biasnetwork 40, and also a coil 4| connected in series with the cathode; thepurpose of this coil will be described later. A pair of tuned outputtransformers 42 and 43 similar to 31 cults (which will be described indetail later), are

to. liottiriue icy-ampli ing alto b ejt p cr gich m r-t c-e el m... apeutcd orch nta rid or smira tc. Z ihe am f -ost mmthctermin l ti-and 2nggcted t q the anode circuit of the valve by a transformer 48. .Itrill,be ee that.

meted-t b fi ntio ly .imthe s meway as the 1-:

valve "I of Fig. 1, the tuned transformers 42 and oo xc tzo dieg. rnectivelyt i hei.e ements, wa U cf this. figur tuned tra s q mexsfl and42 operate as h n pa -filte s a d-.shomd' o'dec i n tenets. a cinte mdia e' freq ency-toas I .Tb t sdtronsfqrmet and #3 s oul m s e Joe. dcsigned to pass an intermediate frea, H,: u l "1. 4. A m 1 two anodes ofthe valve 39. are, each con: nected through the primary winding. of.the. cor:

iesm tl ng tuned-t a sform r to. the positive tor:-

nit al f the .high,.tenion supply,- Athc. neaa tive mma 511 c;which,is,connected.to ground)...,

and the screen grid is connected directlythereto'. v ;mates nding connetions to. the valve-.33 are similar, except that. .two. normally closeda di hWc ecn-intermsed as shown. for. .Rutpo e .toh explained, laterinconnection w thanother.embodim nt.otthcinvcntioncT ey ar not neededfor theiemb diment now being deg1, n u d e m t edv, .p lous t flemtioneddete t r. c rcu t com ris s. do lo cr si ton 53 nds-5 shun ed bycondensers 55 and ,56,-each .connectedhetween oir mndmgtm t ombo e-n(II-7.4.3, an r? Lil. T eii p o -terxn i i ach.secon an! W nd ng is connc t -rou dt r g --tho orr ondi R ie he. ouble stida44.-,,- .Tha 1 o teseob a n a m s eioaflco t- P 191: staple th ou h-t ancc 1 or as. i ad ustnpo ono meter 59 or to the on rol an orth va ves ai fa ta sr t a or. heam li y n Valve-3 s Thisiv lta c. isa

l qq t u tqr 62. owb hsthe. i ut eras oft v ml .th tor t cq ss Q i,,. as.1 1 atr n to me silaan 3 T-l nd ans c 1 ha... .APP .Priat .smq hm Showmay peincludedin theconductors .61;

68 according to" the usual ra t ce; qr vp (res is any amgcont th .incong slgnal H mm eame m ned lim ttov be empl X -.edv i. na out..-

1. r the secondary Wind g'p fir? a d 6,4 orr s ond-Q .i nc tw -d ea; nmitt? i1 int, e, tthandihalf. aof rtheidoubl' diod 44 Theaectifiedz'signalstare-then applied-through' thiresistancet5.1-zandvpotentiometem5 9:. to= theflow fre- Auencyt, mplifier valved!)andrthencexto-the out-- put; ten. .mals :96 and-J41; .aThe motentiometer59 may he :adi ustedctoagive tan iappropriate i output level. At thesame time the double diode 6| protillficsca suitable gain controlvoltage whicheis -ap- 'fi'ntbfl-sltlwcnrinputg grid ofrtheiamplifying 39through the resistances 69 and conductor galvethe;hrgher'rangezareibeing res theSW-tflh-fiizis set in'the upperposition; an heiteqllcnqyrfiiztheroscillator ist set in order to btain;thehigher;z.intermediate frequency (560 's isno generated intheupper-portion F I guenoy; changing :valvest 3:;and' amplified he mperportiom oh the amplifying valve39,

a is rectified in the right hand half of the doumed at on:.Ifhermectified -signal's are applied through resistance;58 randadjustable potentiom etgn ti fl to-the l wirequen'cyvalvedi The gain 7 UH orcthe nppertporticn of the ham 1 {wine hex dc fluis:generatedcby thedouble:

nductor 521 vf-"i aizia'um': in Will hhllSfibfi SEBHrthBJtthGFtWOchannels pros V $51.- byct elvalvesfiitand 39tare never.-employed d odefi l .and applied through resistance 10 and lmu lfilte l sly.flForcthissneason ibis-immaterialthat the upper intermediate ferquencyhappens to t e the l th-harmonic ofgthe ;-lower intermediate firequ mv".TIhe adyantagaof the arrangementsirdihfi? 1- SWEWGhiIlgin\zthesintermediate. ire. 17w detectingricimuits- .is :avoided; the and1 .2 notnloeingpused in: .this r emsalready explained, and being pref-Tim: -1 2: i -W imshown connected inrseries withtthe:

e valve;39 .-may be: USEdrtOfiHtIOdll'Ct-E- toasts negativetteedpaokfor; stabilizingthezoperation: of

or triode portion vof the valve withoutafiecting 1 wer;t1;iode.\;t-lihe,,rwindih AI shoulm oficou-rse -i be; o poledz attheenergy isfedback in negative 9. 1% qzlzprden to obtain". the desiredstabiliza-i smaybe omitted and replaced by onnection wp (5.1%"

di W loyanmother suitablerectifying devices Q H ts.such-astd ztc e ti xQbcteddcd- 33, each such stage congpri m electrode e-errata e.similanyacmhervaive-39.

J smecessary. to 1 provide-lion stand-by: reqeptl ll- Q1 high,equate-y;amplifiers-rand:- selective circuitsadagtedior the range 2tionc glearly the ropil;;A-L-.could.- alternatively be"- tthcatltainstormer 43 int-iwhichtcase the" A edloack will .be applied/to.the upper flofilfi ns ead? ,Q vliomthe zlowerntriode. :1 int-case it:-is not desired to use any negative feedback at l betievident tthatzeither or bothoi the dipdes; may; be replaced by. two separate y umben ofampliftyingstages. tnot shown): ither ,tbeforec or after the :valver25,000 kc. s. for ex- :1 iafigmhrisfisiiselective.andaamplify t rec fleding circuits for the stand-by range 488 to 515 kc. s.

The carrier frequency oscillator 34 is as before, and is adapted for therange covered by the amplifier 3i. A switch 15 allows the antenna 30 tobe connected to either of the elements 31 or 14, and another switch 76,which may be coupled to 15 is provided for disconnecting the oscillator34 when the antenna 30 is connected to the element 14.

Referring now to Fig. 3, the valve 33 should be biassed in such a mannerthat the lower triode portion operates as a simple amplifier for thestand-by range 488 to 515 kc. s., while the upper triode acts as amodulator for the high frequency range as before, and the oscillator 34is set to produce a suitable intermediate frequency such as 1,000 kc.s., to which the transformers 38 and 43 will be tuned. The transformers31 and 42 will of course be tuned for the stand-by range. Selection bythe double diode 44 for both ranges takes place as before.

The advantage of this arrangement is that the operator may set hisapparatus to receive on some frequency in the high frequency range, andby simply operating the two switches 75 and 16 (Fig. 4) he may listen inat any time on the stand-by range without upsetting the normaladjustments of the apparatus for the high frequency range. It is to benoted that the switch 16 disconnects the oscillator 34 when the valve 33is acting as an amplifier for the stand-by range.

It will be seen that the stand-by range is demodulated directly withoutthe introduction of an intermediate frequency, and the two portions ofthe valve 33 perform different functions.

There are, of course, other ways in which the switching may be carriedout. Another method utilizes the switches and 52 included in Fig. 3. InFig. 4 the switch 15 may be omitted and the antenna 39 may bepermanently connected to both of the devices 3| and 14. Then when it isdesired to receive on the high frequency range, the switch 5| (Fig. 3)is opened, thus disabling the lower triode portion of the valve 33, andthe switch 16 (Fig. 4) is left in the position shown. In order to listenin the stand-by range, the switches 52 and 16 are opened, and switch 5|is closed. The upper portion of the valve 33 is now disabled and theoscillator 34 is disconnected. The three switches 5|, 52 and 16 mayclearly be mechanically connected so that all the necessary switching isperformed simultaneously for each range.

Fig. 5 shows an example of a pentagrid valve arranged according to theinvention to produce amplification in each of two entirely separatesignal channels which may be used separately or simultaneously. Thevalve 39 with its tuned transformers 37, 38, 42 and 43 is arrangedpractically in the same way as in Fig. 3 except that the feedback coil4i and the automatic gain control arrangements are omitted.

The first channel comprises an antenna 11 connected to receivingcircuits represented by the block i8 and including ferquency changingmeans supplying a first intermediate frequency to the tuned transformer31. This channel also comprises a detecting circuit 19 connected to thetuned transformer 42 and supplying the detected signals to the receiver80. The second channel comprises the antenna 8|, receiving and frequencychanging circuits 82 supplying a second intermediate frequency to thetuned transformer 38, and a detecting circuit 83 connected to the tunedtransformer 43 and supplying the detected signals to the receiver 84.The two intermediate frequencies (which should preferably be wellseparated, as already explained) are amplified, by the respectiveportions of the pentagrid 39 as previously described, and are separatelydetected and supplied to corresponding receivers. The two signalchannels so obtained are thus quite independent of one another.

Additional stages of amplification (not shown) may be provided by othervalves arranged in the same way as the valve 39.

The circuits represented by the blocks I8, 19, 82 and 83 are not shownin detail, and they may be provided in any well known way.

In Figs. 3 and 5, the tuned transformers 31, 38, 42 and 43 representonly one possible form of selective circuits which may be used forkeeping the two channels separate. Various other types of tuned circuitscould be used, or wave filters of various configurations. What isrequired is that the selective circuits used in one channel shalltransmit the waves corresponding to that channel and shall exclude thosecorresponding to the other channel.

Referring again to the valve 39 shown in Fig. 5,

' which is arranged as a simple amplifier in both channels if the outputsides of the tuned transformers 42 and 43 were connected back to therespective input grids of the valve 39 in place of the transformers 37and 38, instead of being connected to the circuits '8 and 79, as shown,then a double oscillator would result giving two different frequenciesrespectively determined by the tuning of the transformers 42 and 43.

A similar result would be obtained whatever type of selective circuitwere employed, provided, of course, that the poling of the connectionsis such as to produce positive feedback, and that the gain of each ofthe triode portions is sufficient.

Fig. 6 shows an example of a double oscillator designed according tothis principle. The valve 85 is a pentagrid as before. is closed, theanode of the lower triode portion is coupled back to the control grid bymeans of the inductively coupled coils 81 and 88 which are tuned by thecondenser 89, the circuit being completed by the relatively largeblocking condenser 99 and by-pass condenser 9|. In a similar way whenthe switch 92 is closed, the anode of the upper triode portion of thevalve is coupled back to the control grid by means of the inductivelycoupled coils 93 and 94 which are tuned by the condenser 95, the circuitbeing completed by the blocking condenser 96. The upper and lowercontrol grids are connected to the cathode by the high resistances 9!and 98. The high tension supply is intended to be connected to theterminals 99 and l 99 as indicated and the negative terminal I99 may beconnected to ground.

Thus so long as the windings 81 and 88 and the windings 93 and 94,respectively, are poled so that positive feedback is obtained in bothcases, then oscillations at one frequency determined by the tuning ofthe circuit 81, 88, 89 will occur when the switch 86 is closed, and maybe obtained from a coil I ill coupled to the coils 81 and 88. Like wiseif the switch 92 is closed oscillations at another frequency determinedby the tuning of the circuit 93, 94, 95 will occur and may be obtainedfrom a coil I02 coupled to the coils 93 and 94.

The oscillator of Fig. 6 may be conveniently employed in combinationwith Fig. 3 for the reception of continuous wave telegraphy. The outputcoils HM and H12 may be respectively connected to correspondingadditional windings (not shown) When the switch 86 on the transformers42 and 43 of Fig. 3. Assuming that the intermediate frequenciescorresponding to the two continuous wave channels are 40 k. c. s. and560 k. c. s. as before, then the lower and upper triodes in Fig. 6 maybe tuned to oscillate at H and 561 k c. s. for example, giving a beatnote of 1000 cycles with the above mentioned intermediate frequencies.

Then the switches 86 and 92 may be mechanically coupled with the switch3| so that when receiving n the lower channel of Fig. 3 the switch 86 isclosed, and when receiving on the other channel the switch 92 is closed.Another switch (not shown) associated with the oscillator 34 may ifdesired be also mechanically coupled to the switch 3| so that the properlocal carrier oscillator frequency is obtained automatically in eachcase.

It will be evident, of course, that the oscillator of Fig. 6 may bedesigned to produce any desired pair of frequencies.

What is claimed is:

1. An electric wave translating arrangement for a wide band offrequencies comprising a thermionic valve having a cathode, an anode,and at least three grid electrodes mounted in order therebetween, asource of signals of a wide band of frequencies, a switch connected tosaid source and having two contacts connected respectively to the firstand third of said grid electrodes counting from said cathode, and twoselective filter circuits connected respectively to the second of saidgrid electrodes and to said anode, the filter circuit connected to saidsecond grid electrode being designed to pass the lower frequencies ofsaid band of frequencies and to exclude the higher band, the filtercircuit connected to said anode being designed to pass the higherfrequencies of said band of frequencies and to exclude the 1ower band,whereby said switch is positioned to make contact with said first gridcontact for said lower frequencies and with said third grid contact forsaid higher frequencies, said arrangement passing said broad band offrequencies.

2. A translating arrangement according to claim 1, wherein each of saidfilter circuits comprises a band filter composed of a transformer havingtuned primary and secondary windings, each of said filter circuits beingdesigned to exclude frequencies passed by the other of said filters.

3. A translating device according to claim 1, wherein said signal sourceand said switch are connected in series between said cathode and the oneof said grids to which said switch is connected and which furthercomprises a source of a constant frequency connected in series with saidcathode and said signal source for beating said constant frequency withsaid signals.

4. A translating device according to claim 1, which further comprises asource of operating potential for said tube connected in series withsaid cathode and both said filter circuits, whereby said tube amplifiessaid signals.

JOHN DOUGLAS HOLLAND.

REFERENCES CITED The following referenlces are of record in the

